The invention relates to a fused-cast refractory product comprising more than 50% alumina Al2O3 by weight and more than 10% magnesia MgO by weight.
Among refractory products, a distinction is conventionally made between fused-cast products and sintered products.
Unlike sintered products, fused-cast products usually include an intergranular glassy phase linking crystallized grains. The problems posed by sintered products and by fused-cast products, and the technical solutions adopted for solving them, are therefore generally different. A composition developed for manufacturing a sintered product is therefore a priori unable to be used as such for manufacturing a fused-cast product, and vice versa.
Fused-cast products, often called “electrocast” products, are obtained by melting a mixture of appropriate raw materials in an electric arc furnace or by any other technique suitable for these products. The molten material is then cast in a mold, after which the product obtained undergoes a controlled cooling cycle so as to be brought to room temperature without fracturing. This operation is called “annealing” by those skilled in the art. It may also be free cooling, according to the molding technology used.
The fused-cast product Monofrax® L, produced and sold by RHI Monofrax Ltd, has a chemical composition comprising 53.6% Al2O3, 44.9% MgO, 0.44% silica SiO2, 0.23% sodium oxide Na2O and less than 0.35% CaO by weight.
The gasification of coal is a process, known for about fifty years, which currently is undergoing considerable development. Specifically, it allows the production, on the one hand, of syngas (CO, H2)—a clean energy source—and, on the other hand, of basic compounds for the chemical industry starting from very diverse hydrocarbon materials, for example coal, petroleum coke or even heavy oils to be recycled. This process furthermore makes it possible to remove the undesirable components, for example NOx, sulfur or mercury, before any discharge into the atmosphere.
The principle of gasification consists of a controlled partial combustion, in steam and/or oxygen, at a temperature of between about 1150° C. and 1600° C. and under pressure.
There are various types of gasifier: fixed bed gasifier, fluidized bed gasifier and entrained bed gasifier. These gasifiers differ by the way in which the reactants are introduced, the way in which the oxidant-fuel mixture is made, the temperature and pressure conditions and the process for removing the ash or slag, liquid residues resulting from the reaction.
The article entitled “Refractories for Gasification” published in the journal Refractories Applications and News, Volume 8, Number 4, July-August 2003, written by Wade Taber of the Energy Systems Department of the Saint-Gobain Industrial Ceramics Division, describes the structure of an internal lining for a gasifier.
The refractory products of an internal gasifier lining are subjected to thermal cycling, corrosion and erosion by the slag or dry ash and the effect of hot spots where the temperature may typically reach about 1400° C. The erosion and corrosion may especially result in the infiltration, into the pores of the refractory products, of slag compounds or liquefied ash, which, under the effect of the erosion and the thermal cycling, may cause the lining to flake off and finally stop the reactor.
Moreover, the development of gasification of black liquor resulting from the manufacture of paper pulp requires new materials that are capable of withstanding the corrosive action of sodium compounds and, in particular, have a low porosity in order to prevent infiltration of aggressive substances.
To increase the lifetime of refractory linings, researchers have tried to increase its thickness. However, this solution has the drawback of increasing the costs and of reducing the useful volume of the gasifier, and therefore reducing its efficiency.
James P. Bennett, in the article “Refractory liner used in slagging gasifiers” published in the journal Refractories Applications and News, Volume 9, number 5, September-October 2004, pages 20-25, explains that the lifetime of current gasifier refractory linings, in particular in air-cooled systems, is very limited despite their high chromium oxide content. Mentioned in particular is the report by S. J. Clayton, G. J. Stiegel and J. G Wimer “Gasification Technologies, Gasification Markets and Technologies—Present and Future, an Industry Perspective”, US DOE, DOE/FE report 0447, July 2002. The use of products containing chromium has also been abandoned in the case of black liquor gasification.
There is therefore a need for a refractory product having properties tailored to the specific environments encountered inside gasifiers and in particular having a porosity low enough to limit the infiltration of aggressive substances.
The aim of the present invention is to meet this need.